Degradation of oxide varistor ceramics in air atmosphere containing NO2 at elevated temperatures

In this paper, the cyclic fatigue hysteresis of carbon fiber reinforced silicon carbide (C/SiC) and SiC/SiC ceramic–matrix composites with different fiber preforms at room and elevated temperatures is investigated. The evolution of fatigue hysteresis dissipated energy versus applied cycle number for unidirectional C/SiC ( σmax = 240 MPa at room temperature and σmax = 250 MPa at 800℃ in air atmosphere), cross-ply C/SiC ( σmax = 105 MPa at room temperature and 800℃ in air atmosphere), 2D C/SiC ( σmax = 387 and 425 MPa at room temperature), 2.5D C/SiC ( σmax = 180 MPa at room temperature, σmax = 140 MPa at 800℃ in air atmosphere, and σmax = 230 MPa at 600℃ in inert atmosphere), 2D SiC/SiC ( σmax = 130 MPa at 600℃, 800℃, and 1000℃ in inert atmosphere, σmax = 80 MPa at 1000℃ in air atmosphere, σmax = 100 MPa at 1000℃ in steam atmosphere, σmax = 140 MPa at 1200℃ in air and in steam atmospheres, σmax = 90, 120 MPa at 1300℃ in air atmosphere), and 3D SiC/SiC ( σmax = 100 MPa at 1300℃ in air atmosphere) is analyzed. The change rate of the fatigue hysteresis dissipated energy between C/SiC and SiC/SiC composites is compared. The fatigue hysteresis dissipated energy decreases with applied cycle number for unidirectional, and cross-ply C/SiC composite at 800℃ in air, and 2.5D C/SiC composite at 600℃ in inert; and the fatigue hysteresis dissipated energy increases with applied cycle number for 2.5D C/SiC composite at 800℃ in air, 2D SiC/SiC composite at 600℃, and 800℃ in inert.

Download Full-text

Temperature Dependence of High-Cycle Fatigue Behavior of a Ni-Base Single Crystal Superalloy

Materials Science Forum ◽

10.4028/www.scientific.net/msf.546-549.1219 ◽

2007 ◽

Vol 546-549 ◽

pp. 1219-1224 ◽

Cited By ~ 1

Author(s):

Y. Liu ◽

Jin Jiang Yu ◽

Yan Xu ◽

Xiao Feng Sun ◽

Heng Rong Guan ◽

...

Keyword(s):

Single Crystal ◽

High Cycle Fatigue ◽

Elevated Temperatures ◽

Fatigue Behavior ◽

Fatigue Cracks ◽

Testing Temperature ◽

Cycle Fatigue ◽

Air Atmosphere ◽

Ultimate Failure ◽

Increasing Temperature

Smooth specimens of single crystal (SC) superalloy SRR99 with [001] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700°C, 760°C, 850°C and 900°C in air atmosphere. The results demonstrated that conditional fatigue strength reached the maximum at 760°C and decreases with increasing temperature. Analysis on fracture surface showed a trend for cleavage rupture at 850°C and 900°C and ductile rupture at 700°C and 760°C. Fatigue cracks initiated at the surface or subsurface were primarily responsible for the ultimate failure. The influence of testing temperature on fatigue lifetime was studied by examining evolution of the microstructure through SEM observation. With the process of cyclic loading at elevated temperatures, the primary cuboidal γ′ precipitates tended to agglomerate and spheroidized, meanwhile a larger number of secondary γ′ particles were formed in the γ matrix in specimens fatigue tested at 700°C, which would have a significant effect on the high temperature properties.

Download Full-text

Reaction mechanism and kinetics of sulfide copper concentrate oxidation at elevated temperatures

Metallurgical and Materials Engineering ◽

10.30544/320 ◽

2017 ◽

Vol 23 (3) ◽

pp. 267-280 ◽

Cited By ~ 1

Author(s):

Aleksandra Mitovski ◽

Nada Štrbac ◽

Miroslav Sokić ◽

Milan Kragović ◽

Vesna Grekulović

Keyword(s):

Reaction Mechanism ◽

Elevated Temperatures ◽

Oxidation Process ◽

Reaction Rates ◽

Oxidation Products ◽

Arsenic Content ◽

Iron Sulfides ◽

Initial Sample ◽

Copper Concentrate ◽

Air Atmosphere

Sulfide copper concentrate from domestic ore deposit (Bor, Serbia) was subjected to oxidation in the air atmosphere due to a better understanding of reaction mechanism and oxidation of various sulfides present in the copper concentrate at elevated temperatures. Results of the initial sample characterization showed that concentrate is chalcopyrite–enargite-tennantite type, with an increased arsenic content. Characterization of the oxidation products showed the presence of sulfates, oxysulfates, and oxides. Based on predominance area diagrams for Me-S-O systems (Me = Cu, Fe, As) combined with thermal analysis results, the reaction mechanism of the oxidation process was proposed. The reactions which occur in the temperature range 25 – 1000 °C indicate that sulfides are unstable in the oxidative conditions. Sulfides from the initial sample decomposed into binary copper and iron sulfides and volatile arsenic oxides at lower temperatures. Further heating led to oxidation of sulfides into iron oxides and copper sulfates and oxysulfates. At higher temperatures sulfates and oxysulfates decomposed into oxides. Kinetic analysis of the oxidation process was done using Ozawa’s method in the non-isothermal conditions. The values for activation energies showed that the reactions are chemically controlled and the temperature is the most influential parameter on the reaction rates.

Download Full-text

Surface Degradation of Ductile Metal in Elevated Temperature Gas-Particle Streams

Volume 2: Coal, Biomass and Alternative Fuels; Combustion and Fuels; Oil and Gas Applications; Cycle Innovations ◽

10.1115/85-gt-91 ◽

1985 ◽

Author(s):

Alan Levy ◽

Yong-Fa Man

Keyword(s):

Erosion Rate ◽

Loss Rate ◽

Elevated Temperatures ◽

Negative Slope ◽

Material Loss ◽

Loss Mechanism ◽

High Loss ◽

Air Atmosphere ◽

Continuous Scale ◽

Erosion Corrosion

The mechanisms and rates of erosion and combined erosion-corrosion of 9CrlMo steel and 310SS at elevated temperatures were investigated to better understand the behavior of piping steels in fluidized bed combustor environments. Tests were performed in a partially inert gas atmosphere to study erosion behavior and in an air atmosphere to study combined erosion-corrosion behavior. It was determined that the erosion rate remained constant or decreased with temperature in nitrogen until a temperature was reached at which the tensile strength v.s. temperature curve of the alloy markedly changed its negative slope. Above this temperature the erosion rate increased rapidly with temperature. In an erosion-corrosion environment corrosion was the dominant mechanism at all test conditions. At higher temperatures and velocities the material loss mechanism changed from low loss rate chipping of the scale to high loss rate periodic spalling. The continuous scale formed on 9CrlMo steel in air appeared to protect the metal surface, decreasing its loss rate in α=30° tests compared to that of 310SS tested at the same conditions in nitrogen where a continuous scale did not form.

Download Full-text

Electrophoretic Deposition of Nanocrystalline BaTiO3 in Ethanol Medium

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.314.133 ◽

2006 ◽

Vol 314 ◽

pp. 133-140 ◽

Cited By ~ 2

Author(s):

Aydin Dogan ◽

Göktuğ Günkaya ◽

E. Suvaci ◽

Markus Niederberger

Keyword(s):

Electrophoretic Deposition ◽

Elevated Temperatures ◽

Deposition Process ◽

Xrd Analysis ◽

Titanium Isopropoxide ◽

One Pot ◽

Flat Surfaces ◽

Air Atmosphere ◽

Coating Method ◽

Optimal Process Control

Electrophoretic deposition technique is a versatile coating method. With this method not only flat surfaces but also curved or rough surfaces can be coated in various thicknesses. In this study, BaTiO3 nanocrystals were synthesized with a novel nonaqueous method. In a simple one pot reaction process, the metallic barium is directly dissolved in benzyl alcohol at slightly elevated temperatures. After the addition of titanium isopropoxide and subsequent solvothermal treatment nearly spherical BaTiO3 nanocrystals typically 5 nm in diameter were obtained. The stability of the suspension in ethanol and nanocrystalline particle size distribution was measured by Malvern Nano-ZS. The BaTiO3 suspensions were directly used in an electrophoretic deposition process without any additional operation. Pt plated alumina was used as substrate. Various voltages were applied by altering the cathode to anode distance as well as deposition time for optimal process control. Because of the ethanol adsorbed on the surface of the nanoparticles, application of high voltages was possible without causing hydrolysis. The deposited surface coatings were dried in air atmosphere and sintered at different temperatures. SEM, EDX and XRD analysis were employed for the investigation of the coating.

Download Full-text

Investigation of Bi2S3 oxidation process at elevated temperatures in the air atmosphere

Tehnika ◽

10.5937/tehnika2005587s ◽

2020 ◽

Vol 75 (6) ◽

pp. 587-593

Author(s):

Nada Štrbac ◽

Miroslav Sokić ◽

Aleksandra Mitovski ◽

Dejan Gurešić ◽

Kristina Božinović ◽

...

Keyword(s):

Kinetic Analysis ◽

Elevated Temperatures ◽

Oxidation Process ◽

Oxidation Products ◽

X Ray Diffraction ◽

Gibbs Free Energy Change ◽

Stability Diagrams ◽

Air Atmosphere ◽

Different Temperatures ◽

Sulfide Concentrates

Bismuth (III) sulfide has been widely researched in recent years due to its application, but little emphasis has been placed on research regarding its behavior at elevated temperatures. This is of great importance, considering that bismuth, in the form of Bi2S3, is found in copper sulfide concentrates and is considered one of the most harmful components, along with lead, arsenic and antimony. The removal of these substances is one of the basic tasks in the processes of pyrometallurgical extraction of copper, in order to obtain a high purity metal. In order to better understand the behavior of bismuth (III) sulfide during oxidation at elevated temperatures, this paper characterized the synthesized sample of Bi2S3 at room temperature and the oxidation products at 500 °C by X-Ray diffraction (XRD), as well as thermodynamic, thermal and kinetic analysis of the oxidation process of Bi2S3 at elevated temperatures. In order to understand the behavior of Bi2S3 during oxidation and the distribution of oxidation products, phase stability diagrams in the Bi-S-O system were constructed at different temperatures and the values of the Gibbs free energy change at 400 °C and 1000 °C were calculated. Kinetic analysis of Bi2S3 oxidation process was performed by Kissingers method in non-isothermal conditions.

Download Full-text

Air-thermal oxidation of zirconium

JOURNAL OF ENGINEERING & PROCESSING MANAGEMENT ◽

10.7251/jepm1709081k ◽

2017 ◽

Vol 1 (9) ◽

Author(s):

Zagorka Koneska ◽

Ružica Manojlović ◽

Dafinka Stoevska-Gogovska

Keyword(s):

Weight Gain ◽

Oxide Film ◽

Thermal Oxidation ◽

Raman Spectra ◽

Metal Surface ◽

Elevated Temperatures ◽

Good Adhesion ◽

Air Atmosphere ◽

Micro Cracks ◽

Parabolic Rate

Zirconium is one of those few metals which are capable of dissolving relativelylarge quantities of oxygen. When heated in air at elevated temperatures, anoxide layer is built up at the metal surface. The oxidation of mechanicallypolished zirconium was studied in the range 500-900oC in air atmosphere, attemperature intervals of 100oC, for exposure times from 5 min to 1860 min foreach temperature. The weight gain of the oxidized species was the reactedamount of oxygen with zirconium for the formation of the ZrO2. The weight gainand thickness of the oxide film increase with the increasing of the oxidation timeand temperature. The oxidation initially followed a parabolic rate at alltemperatures. At temperatures higher than 700oC oxide “breakaway” appears atthe longest oxidation times. Microstructural investigations have shown that theoxide layers are compact and with good adhesion to the metal surface, but attemperatures over 700oC, radial micro-cracks appear. Raman spectra of theformed oxides at the investigated temperatures are characteristic for monoclinicphase.

Download Full-text

Langasite as Piezoelectric Substrate for Sensors in Harsh Environments: Investigation of Surface Degradation under High-Temperature Air Atmosphere

Sensors ◽

10.3390/s21175978 ◽

2021 ◽

Vol 21 (17) ◽

pp. 5978

Author(s):

Thierry Aubert ◽

Ninel Kokanyan ◽

Omar Elmazria

Keyword(s):

High Temperature ◽

Measurement Errors ◽

Elevated Temperatures ◽

Prolonged Exposure ◽

Acoustic Properties ◽

Annealing Duration ◽

Low Oxygen ◽

Air Atmosphere ◽

Before And After ◽

The Stability

Langasite crystals (LGS) are known for their exceptional piezoelectric properties at high temperatures up to 1000 °C and more. In this respect, many studies have been conducted in order to achieve surface acoustic wave (SAW) sensors based on LGS crystals dedicated to high-temperature operations. Operating temperatures of more than 1000 °C and 600 °C for wired and wireless sensors, respectively, have been reached. These outstanding performances have been obtained under an air atmosphere since LGS crystals are not stable in high-temperature conditions under a low-oxygen atmosphere due to their oxide nature. However, if the stability of bulk LGS crystals under a high-temperature air atmosphere is well established, the surface deterioration under such conditions has been hardly investigated, as most of the papers dedicated to LGS-based SAW sensors are essentially focused on the development of thin film electrodes that are able to withstand very elevated temperatures to be combined with LGS crystals. Yet, any surface modification of the substrate can dramatically change the performance of SAW sensors. Consequently, the aim of this paper is to study the stability of the LGS surface under a high-temperature air environment. To do so, LGS substrates have been annealed in an air atmosphere at temperatures between 800 and 1200 °C and for durations between one week and one month. The morphology, microstructure, and chemical composition of the LGS surface was examined before and after annealing treatments by numerous and complementary methods, while the surface acoustic properties have been probed by SAW measurements. These investigations reveal that depending on both the temperature and the annealing duration, many defects with a corolla-like shape appear at the surface of LGS crystals in high-temperature prolonged exposure in an air atmosphere. These defects are related to the formation of a new phase, likely an oxiapatite ternary compound, the chemical formula of which is La14GaxSi9−xO39−x/2. These defects are located on the surface and penetrate into the depth of the sample by no more than 1–2 microns. However, SAW measurements show that the surface acoustic properties are modified by the high-temperature exposure at a larger deepness of at least several tens of microns. These perturbations of the LGS surface acoustic properties could induce, in the case of LGS-based SAW sensors operating in the 434 MHz ISM band, temperature measurement errors around 10 °C.

Download Full-text

Characterization of Rare-Earth Fluoride Anti-Stokes Phosphors by Scanning arid Transmission Electron Microscopy

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100064761 ◽

1971 ◽

Vol 29 ◽

pp. 142-143

Author(s):

N. M. P. Low ◽

L. E. Brosselard

Keyword(s):

Electron Microscopy ◽

Rare Earth ◽

Elevated Temperatures ◽

Stoichiometric Composition ◽

Rare Earth Ions ◽

Crystalline Solid ◽

Precipitation Process ◽

Rare Earth Fluoride ◽

Earth Fluoride ◽

Rare Earth Fluorides

There has been considerable interest over the past several years in materials capable of converting infrared radiation to visible light by means of sequential excitation in two or more steps. Several rare-earth trifluorides (LaF3, YF3, GdF3, and LuF3) containing a small amount of other trivalent rare-earth ions (Yb3+ and Er3+, or Ho3+, or Tm3+) have been found to exhibit such phenomenon. The methods of preparation of these rare-earth fluorides in the crystalline solid form generally involve a co-precipitation process and a subsequent solid state reaction at elevated temperatures. This investigation was undertaken to examine the morphological features of both the precipitated and the thermally treated fluoride powders by both transmission and scanning electron microscopy.Rare-earth oxides of stoichiometric composition were dissolved in nitric acid and the mixed rare-earth fluoride was then coprecipitated out as fine granules by the addition of excess hydrofluoric acid. The precipitated rare-earth fluorides were washed with water, separated from the aqueous solution, and oven-dried.

Download Full-text